Surface aeration impeller designs

ABSTRACT

A surface aeration impeller for use in a liquid filled tank. The impeller is rotatable about an axis perpendicular to the static liquid surface. The impeller has a plurality of blades mounted on the underside of a disc or disc-like surface. Each blade has a multi-faceted or curved geometry ranging from vertical at the point of attachment to the disc to partially inclined at the bottom. The blades are spaced circumferentially about the axis and are disposed at acute angles to radial lines from the axis of rotation of the impeller. The lower portions of the blades, which are inclined but non-vertical, are positioned at or below the static liquid surface. When the impeller is rotated, the lower portion pumps the liquid up onto the vertical portion of the blades where the liquid is discharged into a spray umbrella in a direction upwardly and outwardly away from the impeller. The design of the invention produces substantially higher oxygen transfer efficiency and overall liquid pumping rates than prior art designs and is particularly useful in the aeration of sewage and other wastewater.

RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 10/244,349,filed on Sep. 16, 2002, now U.S. Pat. No. 6,715,912 issued on Apr. 6,2004, which is a Continuation In Part of application Ser. No. 09/895,418filed Jul. 2, 2001, now U.S. Pat. No. 6,464,384, which is a Continuationof application Ser. No. 09/358,502, filed Jul. 21, 1999, now abandoned,which is a Continuation of application Ser. No. 09/162,088 filed Sep.28, 1998, now U.S. Pat. No. 5,972,661.

FIELD OF THE INVENTION

The present invention relates generally to rotating impellers. Morespecifically, the invention relates to surface aeration impellers thatrotate on a vertical axis near the surface of a body of liquid in a tankcausing liquid to be sprayed into the gas above the liquid and gas to beentrained into the liquid by the liquid spray impinging onto the liquidsurface.

BACKGROUND OF THE INVENTION

In a number of industrial processes it is desirable to enhance the masstransfer of a gas into a liquid. Much of this need results frombiochemical oxidation processes which use aerobic microbes. Aerobicmicrobes are employed because they are able to convert a raw materialinto a higher value material. Some examples include aerobic fermentationprocesses for manufacturing fragrances, flavors, and pharmaceuticalcomponents. Perhaps an even more important process is the aeration ofsewage and other wastewater streams. What all these processes usingaerobic microbes have in common is the need for oxygen to be dissolvedinto the liquid in order for the microbes to be able to convert the rawmaterial into the desired result. Since the microbes work mostefficiently when there is an adequate level of dissolved oxygenavailable in the liquid, it is typically desirable to transferadditional amounts of oxygen or air into the liquid. This can beaccomplished in a number of ways but the two most common techniques aregas sparging and surface aeration. In a gas sparging procedure, a gas(e.g. air or oxygen) is bubbled through the liquid in a manner thatincreases the amount of dissolved oxygen in the liquid. In contrast,surface aeration uses an impeller located at the surface of the liquidto agitate or spray the liquid into the gas. The spray subsequentlyimpinges on the liquid surface which also entrains gas into the liquidsurface.

One of the basic procedures for the treatment of sewage and otherwastewater streams is known as the activated sludge process, which wasdiscovered or invented more than seventy years ago. It is a biochemicaltype of reaction, involving the mass transfer of oxygen into water, andthen the transfer and use of that dissolved oxygen to support the growthof aerobic microorganisms suspended in the water. These microorganisms,known as the biomass, oxidize the organic materials in the wastewater indifferent ways to eliminate the biochemical oxygen demand (BOD) of thewastewater.

The original activated sludge process involved introducing air from ablower through various forms of diffuser devices located in the bottomof the aeration tank or basin. These devices generally have lowoxygen-transfer efficiency and poor solids-suspension characteristics.More than forty years ago, a different approach was taken to aeration inthe activated sludge process. This different approach was known asmechanical surface aeration. This technique made use of a mechanicalagitator operating at the liquid surface to throw or spray liquid intothe air and to induce entrainment of air into the liquid surface,without the use of a compressor and the diffusers. Since that time, afairly large number of different designs for surface aeration impellershave been introduced, both for the purpose of increasing theoxygen-transfer efficiency and also, secondarily, if possible, toimprove liquid mixing and solids suspension. The problem of solidssuspension, however, has an obvious limitation because of the remotenessof the surface aeration impeller from the tank bottom where the biomasssolids tend to settle if the bulk liquid in the tank is not adequatelymixed.

The standard measure of aeration efficiency is the number of pounds ofoxygen transferred into the liquid per hour per horsepower of energyused to operate the aeration system. This measure is known as theStandard Aeration Efficiency (SAE). The SAE for current state of the artsurface aeration devices ranges from about 2.0 to about 3.3 pounds ofoxygen per hour per horsepower in the larger aerator sizes. In smallersizes, the efficiency values can be somewhat higher. Since wastewatertreatment plants are pure cost centers (i.e. they do not sell a product)and since electric power is one of the main operating costs in such aplant, the oxygen-transfer efficiency performance of such aerators isextremely important, especially in the larger plants. This need has ledto a number of attempts at producing surface aeration impeller designswith greater oxygen transfer efficiency.

Typical of state of the art surface aeration impellers are those shownin U.S. Pat. No. 3,479,017 to Thikotter; U.S. Pat. Nos. 3,576,316 and3,610,590 to Kaelin; and U.S. Pat. No. 3,741,682 to Robertson; U.S. Pat.No. 4,066,383 to Lakin; U.S. Pat. No. 4,074,953 to Budde et al.; U.S.Pat. No. 4,151,231 to Austin; U.S. Pat. No. 4,334,826 to Connolly etal.; U.S. Pat. No. 5,522,989 to Hove; and U.S. Pat. No. 5,988,604 toMcWhirter. All of these patents are incorporated herein in theirentirety.

Thikotter discloses a surface aeration impeller for use in an activatedsludge process. Thikotter's aerator comprises a flat, circular impellerdisc having a plurality of impeller blades depending from theundersurface of the disc. The blades are generally flat, positionedradially and have a height that decreases from its inner edge to itsouter edge. This design principally focuses on spraying the liquid anddoes not provide much up-pumping action or mixing of the tank liquidcontent resulting in relatively low efficiency of the system. Robertsonand Austin also disclose surface aeration impellers having multipleblades located on the underside of a disc. Their blades are radial orapproximately radial and generally flat but have a horizontal platesecured to the lower edge of each blade. Again, these designs primarilyfocus on throwing or spraying of the liquid and do not provide muchup-pumping action and mixing of the body of liquid in the tank.

Unlike Thikotter, Roberston, and Austin, Lakin and Connolly disclosevarious forms of surface aeration impellers having primarily verticallycurved blades. Most seem to have multiple blades on a disc-shapedmounting member. Kaelin and Budde et al. also teach surface aeratordesigns. The blades of Budde et al. are radial and Kaelin show otherdesigns representative of the state of the art. The design of Budde etal. does not provide much mixing action and Kaelin in addition suffersfrom the disadvantage of being difficult to manufacture.

Hove teaches a device and method for aerating wastewater. The device hasmultiple blades positioned on a disc-shaped mounting member. The bladesappear to be entirely radial. Hove's blades are unique compared with theabove patents in that they are located both above and below thedisc-shaped mounting member.

McWhirter '604 teaches a surface aeration impeller that is an axial flowimpeller that may have either pitched blade turbine or airfoil shapedblades. The blades of the McWhirter patent are not mounted to theunderside of a disc-shaped mounting member Additionally, while the uppersection of the '604 blades are not strictly radial, the lower section isradial (at least at one point). This impeller does provide someup-pumping and mixing action but still leaves room for improved liquidpumping and oxygen transfer efficiency.

Although such surface aeration devices as discussed above havefunctioned in a generally satisfactory manner, problems have beenexperienced with excessive splashing and misting, insufficient liquidpumping, mixing and circulation, and clogging of the impellers duringoperation. Accordingly, there continues to be a need for improveddesigns that increase the efficiency of the aeration process and/oraddress some of these problems. In particular, surface aeration impellerdesigns and operational characteristics that increase the oxygentransfer efficiency into the liquid and thereby reduce operating costsare especially desirable.

Many of the limitations associated with prior art surface aeratorimpeller designs result from an insufficient understanding of thefundamental mechanisms and fluid dynamics of surface aeration. Thecurrent state-of-the-art oxygen mass transfer analysis for surfaceaerators is essentially limited to the simple, idealized model employedin the ASCE Standard for the Measurement of Oxygen Transfer in CleanWater. This oversimplified and limited model has been used for decadesto characterize the oxygen mass transfer performance of surfaceaerators. A more realistic and rigorous model has been developed byMcWhirter et al. in “Oxygen Mass Transfer Fundamentals of SurfaceAerators”, Ind. Eng. Chem. Res. 34, 2644-2654, 1995. This mechanisticmodel provides a more physically realistic description of the actualoxygen transfer mechanisms of surface aerators and separates the oxygenmass transfer process into two distinct zones: a liquid spray masstransfer zone and a surface reaeration mass transfer zone.

These two distinctly different mechanisms or zones are created by allgeneric types of mechanical surface aerators. The liquid spray masstransfer zone 11 is created in the immediate gas space surrounding theperiphery of the surface aeration impeller where the liquid isdischarged into the surrounding gas at high velocity. The surfacereaeration mass transfer zone 13 exists primarily outside the sprayumbrella and in the bulk liquid near the surface in the area that iscircumferential to the periphery of the liquid spray mass transfer zone.The two zones are indicated in FIG. 4. The liquid spray mass transferzone can be reasonably characterized and modeled as a single-stagegas-liquid contacting zone wherein the liquid is dispersed into avirtually infinite, continuous gas phase of constant gas compositionabove the liquid surface. In contrast, the mechanism in the surfacereaeration mass transfer zone is predominately characterized by oxygentransfer to a highly turbulent liquid surface containing entrained gasfrom the gas phase above the liquid surface. As the liquid spray zoneimpinges on the liquid surface of the tank, substantial gas bubbleentrainment into the surface is accomplished and a “white-water” effectis produced at the periphery of the liquid spray impingement on thesurface of the tank liquid. The surface reaeration mass transfer zonealso includes the oxygen transfer to the highly turbulent liquid surfacebeneath the spray umbrella and thus includes all oxygen transfer to thesurface liquid due to bubble entrainment and contact of the highlyturbulent liquid surface with the gas above the liquid surface.

In contract to generally perceived prior opinion regarding the primaryoxygen transfer mechanism of surface aerators, the present inventorshave quantitatively shown that about two-thirds of the oxygen transferof surface aerators occurs in the surface reaeration mass transfer zoneand only about one-third in the liquid spray mass transfer zone. Thissuggests that impeller designs that enhance oxygen transfer in thesurface reaeration zone (e.g. by increasing turbulence and volume flowrates) may have a greater overall effect on the total oxygen transfer ofthe system than impeller designs that focus primarily on increasingoxygen transfer in the spray zone (e.g. by improving spraycharacteristics like height and distance). Thus, a greater understandingof the oxygen mass transfer mechanisms in surface aerators has allowedthe present inventors to independently analyze the oxygen transferprocess within these two distinctively separate mass transfer zonesleading to the improved surface aerator impeller designs as disclosed inthis application. These new designs pump more liquid per unit ofhorsepower input through the liquid spray mass transfer zone and intothe surface reaeration zone and thereby maximize the total oxygen masstransfer efficiency of the overall surface aeration system.

Accordingly, the following are selected objects of various embodimentsof the present invention:

It is an object of the present invention to provide an improved surfaceaeration impeller having improved gas transfer rates into the liquidparticularly in the surface reaeration mass transfer zone of the system.

It is also an object of the present invention to enhance turbulence andgas entrainment at the liquid surface created by the liquid spray of asurface aeration system.

It is an object of the present invention to provide an improved surfaceaeration impeller having reduced torque and increased rotational speedleading to reduced costs for motor and gear transmission equipment torotate the impeller.

It is also an object of the present invention to provide an improvedimpeller design having increased liquid pumping capacity and efficiency.

SUMMARY OF THE INVENTION

The invention is an improved surface aeration impeller for use in aliquid filled tank that has a free liquid surface and an enclosed oropen gas space above the liquid surface in the tank. The impeller isrotatable about an axis perpendicular to the static liquid surface. Theimpeller has a plurality of blades mounted on the underside of a disc ordisc-like surface. Each blade has a multi-faceted or curved geometryranging from vertical at the point of attachment to the disc topartially inclined at the bottom. The blades are spacedcircumferentially about the axis and are disposed radially or at acuteangles to radial lines from the axis of rotation of the impeller. Thelower portions of the blades, which are less inclined or less verticalthan the upper portions, are positioned below the static liquid surface.When the impeller is rotated, the lower portion of the impeller bladepumps the liquid up onto the vertical portion of the blades where theliquid is discharged into a spray umbrella in a direction upwardly fromthe static liquid surface and outwardly away from the rotating impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a preferred impeller design according tothe present invention.

FIG. 2 shows an isometric view of an impeller in accordance with thisinvention.

FIG. 3(A) is a profile view of a single blade with an endcap on thetrailing edge.

FIG. 3(B) shows the profile of a curved blade used in one embodiment ofthe present invention.

FIG. 4 shows the surface aeration impeller in operation in a tank.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, approximately two-thirds of the oxygen transfer in asurface aeration system occurs in the surface reaeration mass transferzone 13 while only about one-third occurs in the liquid spray masstransfer zone 11. Further, maximum efficiency of a surface aerationsystem is not maximized by simply increasing the discharge velocity ordistance of travel of liquid spray in the liquid spray mass transferzone as many prior art designs have assumed. This discovery has led thepresent inventors to focus on surface aerator designs that maximize thetotal oxygen transfer efficiency in both mass transfer zones with aparticular emphasis on the surface reaeration mass transfer zone. Thisfocus has led to surface aerator designs that operate significantlydifferent that most prior art designs. In the present invention, thedischarge velocity of the spray from the surface aeration impeller ismuch lower than most state-of-the-art surface aeration impellers. Thisresults in a liquid spray that does not travel as high or as far ascurrent commercial designs. For example, in preferred embodiments of thepresent invention the liquid spray travels only about 8 to 12 feet fromthe tip of the aerator impeller whereas current state-of-the-art surfaceaerators operate with a spray distance of about 15 to 18 feet or morefrom the tip of the impeller. However, while the spray of the presentinvention travels a shorter distance, much more liquid is pumped throughthe liquid spray mass transfer zone per unit of horse power input. Thisis a result of the lower discharge velocity of the liquid spray from thetip of the impeller. The increased liquid flow also creates much moreliquid flow and much more turbulence in the surface reaeration masstransfer zone thus greatly increasing the oxygen transfer rate in thereaeration zone. This oxygen transfer increase in the surface reaerationzone more than compensates for any reduction in oxygen transfer ratewithin the liquid spray zone. Accordingly, the surface aerationimpellers of the present invention are designed in a way that maximizethe volume of liquid flow through the liquid spray and surfacereaeration zones per unit of power input. This result is accomplished bydramatically increasing the up-pumping capability of the surfaceaeration impeller.

Thus, the surface aerator designs of the present invention have at leastfour primary advantages that distinguish them over the prior art. Thesefour primary advantages are:

1. The invention provides more liquid pumping and the spraying of moreliquid per unit of horsepower.

2. The invention provides higher oxygen transfer energy efficiency(SAE).

3. The invention provides better overall tank mixing and higher tankbottom velocities for improved biomass solids suspension.

4. The invention operates at higher speed and lower torque which reducesthe equipment cost (gear reducer) while simultaneously providing all ofthe above advantages.

Referring to the Figures, there is shown in FIG. 1 a top view of animproved surface aeration impeller according to the present invention.The impeller has a plurality of vertically extending blades 2 attachedto the underside of a rotatable disc or disc-like mounting member 1.Each blade in the embodiment shown in FIG. 1 is disposed at an angle (α)of approximately 30-38° to a successive, circumferentially spaced radialline around the axis 3 of the impeller. In the example shown in FIG. 1there are eight blades spaced 45° apart. The blades 2 are more clearlyshown in FIG. 2 which is an isometric view of the impeller. These bladeshave substantially vertical portions 6 at the upward sections thereof.The blades 2 also have a non-vertical and non-horizontal lower section 7which extends downwardly and outwardly in the direction of rotation ofthe impeller. This downwardly direction forms angle β with thehorizontal as shown in FIG. 3(A). The lower portion 7 of the blades actsas up-pumping pitched blade turbines to provide a high volume of liquidflow to the vertical upper portion 6 of the turbine blade which createsthe liquid spray umbrella and liquid spray mass transfer zone 11.

The blades 2 in the present invention consist of at least two sectionsas shown in FIG. 2: (1) the generally vertical upper portion 6 and (2)the non-vertical but inclined lower portion 7. In FIG. 3 a thirdsection, the top or mounting section 8, is also shown, but is optional.This top section is generally horizontal and contains holes for bolting10 through corresponding holes in the mounting disc 1. This section isoptional as other means of mounting the blades to the disc are possible.For example, the vertical section 6 could be directly welded to themounting disc 1 or the vertical section 6 could be mounted directly to avertical flange on the mounting disc. These types of blades are similarin shape to those on pitched blade turbine mixing impellers.

For ease of manufacturing and mounting, the inventors have found that agenerally rectangular shape for all of these sections works well, thoughother shapes are certainly useable. In a preferred embodiment of theinvention, the blades are made from a single rectangular piece of metalthat has been creased in two positions. One crease is at a 90-degreeangle and occurs near the top edge of the blade to provide thehorizontal top portion 8 for easy mounting to the underside of themounting disc 1 and a substantially vertical upper section 6. The secondcrease on this embodiment occurs approximately two-thirds tothree-fourths of the way down the length of the entire rectangular pieceof metal. This crease provides for the downward and outwardly (in thedirection of rotation) extending lower section of the blade 7. Thesecond crease forms angle β shown in FIG. 3(A). The angle β is fromabout 20° to about 60°, preferably about 30° to 50°, and most preferablyis about 35 to 45°.

In a preferred embodiment of the invention the point at which the uppersection of the blades meets the mounting member is a straight line (i.e.the upper section of the blades are straight in the horizontal plane).In another preferred embodiment, all sections of the blades are planer(e.g. rectangular or trapezoid), and are thus non-curved. Also the outeredge of the upper section is typically contiguous with the outer edge ofthe disc-shaped mounting member. While the inventors have foundrectangular shaped blades most desirable, other shapes are useablewithout diverting from the spirit of the invention. It is important forthe blades to begin at the top with a substantially vertical section andend with an outwardly facing (in the direction of rotation) non-verticalsection that will lie at least partially under the liquid surface. Theincline and size of this lower portion is such that it is sufficient toprovide a substantial amount of upward pumping flow of liquid onto thevertical section when the impeller is rotated. These requirements can bemet with the two-section blade described above as well as by amulti-sectioned (more than two) blade and a continuously curved blade asshown in FIG. 3B. Such continuously curved blades can be termed“airfoil” shaped as described in U.S. Pat. No. 5,988,604, especiallyFIG. 6 (incorporated by reference). The blades of the invention (bothcurved and non-curved) preferably have an approximately constant width Walong their entire length. Such blades can be made relatively easilyfrom a single rectangular piece of material (e.g. stainless steel).

The number of blades on the surface aeration impeller of the presentinvention is generally in the range of about 6 to 12. The optimal numberof blades will depend on the specific application, however, smallerdiameter impellers will generally have fewer blades and larger diameterimpellers typically have 8 or more blades. In preferred embodiments thenumber of blades is about 6-8 and in an even more preferred embodimentthere are exactly 8 blades.

The positioning of the blades is important but can also varyconsiderably. The inventors have found that positioning the bladesradially under the disc-shaped mounting member produces a surfaceaeration impeller that out performs all prior art designs. However, theinventors have also discovered that positioning the bladesnon-radially—i.e. they do not project radially outward from the axisperpendicular to the static liquid surface—produces a surface aerationimpeller with even greater liquid pumping capability and oxygen transferefficiency. In this non-radial embodiment, the inner edge of thevertical section of the blade is pushed forward in the direction ofrotation forming a non-zero angle (α) where a is defined as the anglebetween a radial line (through the outer edge of the vertical section)and the top edge of the vertical section 6 of the blade (see FIG. 1).This angle is typically between 20° and 60°, preferably between about25° and 50° and most preferably is about 30-45°. Another way ofcharacterizing the positioning of the blades is that they are “sweptback” or “off-axis” (i.e. non-radial). It is worth noting that in thenon-radial version of the present invention there are no imaginaryradial lines that lie on the surface of any blade section. In otherwords, there are no lines lying on the surface of any blade sectionwhich are also radial lines.

The size of the blades may also vary considerably. Referring to thefigures, the width W of the blades are within the range of about 0.1 to0.4 the diameter d of the disc. Preferably W is less than ⅓ d and mostpreferably is about 0.2 to 0.3 d. The height H of the vertical sectionof the blades are within the range of 0.05-0.25 d, preferably 0.1-0.2 d.The length L of the lower section of the blade is typically less thanthe height of the vertical section. Length L can be from 0.03-0.2 d,preferably less than 0.1 d or about 0.05 d. Finally, the width T of theoptional top section 8 for mounting onto the disc is not critical aslong as it allows for adequate mounting, for example by bolts.

The blades of the invention have an optional additional segment known asan endcap. The endcap 9 is shown in FIGS. 2 and 3(A). The endcap is arelatively flat geometric piece positioned essentially perpendicular tothe vertical section 6 and connects the outer or trailing edges of boththe vertical section 6 and the lower section 7. While the exact shape ofthe endcap can vary widely, the critical feature of the endcap is thatit prevents liquid from flowing or “sliding” off the trailing edge ofthe blades below the vertical section 6 and simultaneously enhances theuplifting or up-pumping capability of the impeller. The inventors havefound that an endcap can significantly increase the power delivered andsimultaneously increase the standard aeration efficiency as the examplesbelow demonstrate.

The blades 2 of the invention are mounted on the underside of a disc 1or a disc-like mounting member for mounting onto a shaft 4 that providesaxial rotation. The disc provides a convenient method for positioningthe blades radially or at an acute angle α as described above. The termdisc-like is meant to include any rotatable mounting member having atleast a top surface and a bottom surface and capable of attaching to thevertical section of the blades radially or at an angle a on a bottomsurface. Included in the term “disc-like” are discs with a saw-toothedshaped edge and spoke and ring type structures.

Means for attaching the entire impeller (disc and blades) to the shaftis not strictly part of the present invention as such means are wellknown to those skilled in the art of impellers. In a preferredembodiment, the mounting member is substantially a disc with a hole inthe center for receiving and connecting to a rotatable shaft 4 using anattachment means 12 which is attached to the disc with bolts 5 and tothe shaft with pins.

The overall diameter of the impellers according to the invention willdepend on the specific application. In the case of sewage or wastewateraeration, typical diameters will be from about 50 to 100 inches. Inother applications, the diameter could be much smaller, especially ifthe tank size is smaller. The size of the impeller is largely determinedby the power required to meet the specific process requirements (i.e.the oxygen transfer rate) but can also be influenced by the size andconfiguration of the tank in which it is employed.

EXAMPLES

Impellers substantially as shown in FIG. 1 were made and tested in a 49feet by 49 feet square tank containing about 17 feet of static liquidwhich corresponds to about 320,000 gallons of water. The test involvedmounting the impeller on a vertical shaft connected to a power sourceand gear reduction means. All the impellers used in the examples belowcontained 8 blades and the overall impeller diameter was 76.25 inches.Additionally, all blades tested had a width W of 20.5 inches and anupper/vertical section height H of 12.5 inches. The horsepower used inthe examples ranged from about 30 to 85 HP. The primary variables were:(1) the “off-axis” angle α, (2) the inclined lower section angle β, (3)liquid submergence, where submergence is defined as the static liquidlevel in inches above the intersection of the vertical and lowersections of the blades, (4) length L of the lower section 7, and (5) thepresence or not of an endcap 9.

Results were primarily determined by calculation of the standardaeration efficiency (SAE) where SAE is defined as the number of poundsof oxygen transferred into the liquid per hour per horsepower of energyused to operate the aeration system. These tests and calculations weremade by using the ASCE standard procedure for determining the SOTR(standard oxygen transfer rate) at 20° C. liquid temperature and 1 atmpressure. The results shown for more than one run are given as theaverage SAE for all runs.

Example 1

This example illustrates one embodiment of the invention with animpeller according to FIG. 1 having a equal to 30°, β equal to 30° and ablade with dimensions h=12.5, w=20.5, and l=12.0 inches. The blade alsodoes not have an endcap. The results (in SAE) show very good efficiencywith some effect of operating the impeller at various submergencelevels.

α β l Submergence Endcap? # Runs SAE 30° 30° 12 in 1.0 in No 2 2.43 30°30° 12 in 3.0 in No 1 2.74 30° 30° 12 in 5.0 in No 2 2.92

Example 2

This example uses the same impeller as demonstrated in Example 1 withthe addition of an endcap. The top of the endcap was approximately oneinch above the crease defining the intersection of the upper and lowersections of the blades. The results (in SAE) show that there is littleeffect in operating this embodiment of the impeller at varioussubmergence levels. The SAE results clearly show the dramaticimprovement in oxygen transfer efficiency possible with the use of theendcap.

α β l Submergence Endcap? # Runs SAE 30° 30° 12 in 0.0 in Yes 3 3.39 30°30° 12 in 4.0 in Yes 3 3.40 30° 30° 12 in 7.0 in Yes 2 3.32

Example 3

This example uses the same impeller as demonstrated in Example 2 withthe exception that the length of the lower section was reduced from 12inches to 8 inches. The results (in SAE) show improved efficiency overprior art designs currently advertised with SAE up to about 3.5. The SAEresults also clearly show that a smaller 8 inch lower blade sectionlength gives higher transfer efficiencies than a 12 inch section forthis configuration.

α β l Submergence Endcap? # Runs SAE 30° 30° 8 in 0.0 in Yes 3 3.56 30°30° 8 in 2.5 in Yes 1 3.78 30° 30° 8 in 5.5 in Yes 3 3.79 30° 30° 8 in7.8 in Yes 1 4.11

Example 4

This example is similar to Example 1 except that the lower sectioninclination angle β is increased to 45° and the length of the lowersection 7 of the blade is reduced to 7 inches. The results (in SAE) aresignificantly improved over Example 1 teaching that in thisconfiguration a larger β and shorter lower section l provide increasedoxygen transfer efficiency. Again this example suggests a general trendof increasing oxygen transfer efficiency with increasing submergencevalues.

α β l Submergence Endcap? # Runs SAE 30° 45° 7 in 4.0 in No 3 3.66 30°45° 7 in 6.0 in No 2 3.97 30° 45° 7 in 7.5 in No 3 4.02 30° 45° 7 in 9.5in No 1 4.09

Example 5

This example is the same as Example 4 with the additional of an endcaphaving its top edge 1 inch above the crease where the vertical and lowersections meet. The results again are generally excellent with SAE above4. The addition of an endcap shows some improvement in oxygen transferefficiency compared with the corresponding example without an endcap.

α β l Submergence Endcap? # Runs SAE 30° 45° 7 in 7.5 in Yes 1 3.46 30°45° 7 in 7.9 in Yes 1 4.20 30° 45° 7 in 8.6 in Yes 1 4.28 30° 45° 7 in9.0 in Yes 2 4.35

Example 6

This example is similar to Example 5 except that the lower blade lengthl was decreased to 4 inches. This impeller also gave excellentefficiency values consistently above 4.0 for various submergence values.

α B l Submergence Endcap? # Runs SAE 30° 45° 4 in 7.4 in Yes 1 3.97 30°45° 4 in 8.4 in Yes 1 4.26 30° 45° 4 in 9.5 in Yes 1 4.34 30° 45° 4 in10.2 in  Yes 1 4.20

Example 7

The impeller used in this example is the same as that used in Example 6except that the “off-axis” angle α was changed to 38° instead of 30°.This impeller also gave excellent efficiency values which weresignificantly and consistently above 4.0 for most submergence levels.

α B l Submergence Endcap? # Runs SAE 38° 45° 4 in 7.0 in Yes 2 4.00 38°45° 4 in 8.5 in Yes 1 4.10 38° 45° 4 in 9.0 in Yes 1 4.21 38° 45° 4 in10.8 in  Yes 1 4.31 38° 45° 4 in 12.3 in  Yes 1 4.23

Example 8

The impeller used in this example was the same as Example 7 except thatthe blades were positioned radially. That is the top edge of the uppervertical section was connected to the underside of the disc mountingmember in a radial manner. These results suggest that the radialembodiment of the invention can produce SAEs better than the beststate-of-the-art results of about 3.3 SAE. However, the radialembodiment does not perform as well as the comparable non-radialimpeller embodiments described above.

α B l Submergence Endcap? # Runs SAE 0° 45° 4 in 2.5-4.0 in Yes 3 3.570° 45° 4 in 6.0-6.5 in Yes 2 3.80 0° 45° 4 in 9.0-9.5 in Yes 3 3.77 0°45° 4 in 10.5-11.5 in  Yes 3 3.33

These examples dramatically demonstrate the improved oxygen transferefficiency of the present invention. State-of-the-art surface aerationimpeller designs produce standard aeration efficiencies of about 3.3over the same range of operating conditions used herein while thepresent invention consistently produces standard aeration efficiencieswell above 3.3 and well above 4.0 for certain non-radial embodiments ofthe invention. Additionally, the present inventors have confirmed thehigher pumping capacity performance of the invention compared with priorart surface aeration impeller designs. With the present impeller designliquid flow velocities throughout the aeration tank are significantlyincreased. This improves overall bulk liquid mixing and can eveneliminate the need for mixing impellers near the bottom of a tank insome applications.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various alterations in form and detail maybe made therein without departing from the spirit and scope of theinvention. In particular, while the invention illustrated by the figuresshows a specific position, size, and shape of the blades on the impellerof the invention, these parameters may be varied within the scope of theinvention as described herein. Further, the means of attaching theblades to an axially mountable member to provide for axial rotation ofthe impeller can vary considerably and is not limited by the preferredembodiments described herein and depicted in the figures.

Additionally, while this application generally addresses the use ofsurface aeration impellers in the treatment of wastewater, the use ofsuch impellers are by no means limited to this application. Surfaceaeration impellers like those of the present invention can be used in avariety of industrial applications where improved aeration is desirable.One particular example in addition to sewage treatment is aeration inbio-reaction processes. These processes include fermentation bycirculating slurries containing microbes and growth media. The presentinvention enables improved oxygenation and mixing of such liquids topromote the fermentation process.

1. A surface aeration impeller designed to rotate about an axisperpendicular to a static liquid surface, said impeller comprising aplurality of blades attached to a mounting member, said mounting memberbeing generally disc-shaped, having top and bottom surfaces, and beingmountable on a shaft for rotation about said axis; wherein said bladescomprise an upper generally vertical section and a lower inclinednon-vertical section that extends downwardly and outwardly in thedirection of rotation, and wherein the height of the outer edge of saidupper section of said blades is between 0.05 and 0.25 d, where d is thediameter of the impeller.
 2. The surface aeration impeller according toclaim 1; wherein the height of the outer edge of said lower section ofsaid blades is between 0.03 and 0.2 d.
 3. The surface aeration impelleraccording to claim 1 having from 6 to 12 blades.
 4. The surface aerationimpeller according to claim 3 having about 8 blades.
 5. The surfaceaeration impeller according to claim 1 wherein said blades additionallycontain an endcap.
 6. A surface aeration impeller designed to rotateabout an axis perpendicular to a static liquid surface, said impellercomprising a plurality of blades attached to a mounting member, saidmounting member being generally disc-shaped, having top and bottomsurfaces, and being mountable on a shaft for rotation about said axis;wherein said blades comprise an upper generally vertical section and alower inclined non-vertical section that extends downwardly andoutwardly in the direction of rotation, and wherein the height of theouter edge of said lower section of said blades is between 0.03 and 0.2d, where d is the diameter of the impeller.
 7. The surface aerationimpeller according to claim 1 or 6 wherein the lower section of eachblade forms an angle β with respect to the horizontal wherein β is fromabout 20° to 60°.
 8. The surface aeration impeller according to claim 1or 6 wherein the upper generally vertical section of the blades ismounted underneath the disc shaped mounting member non-radially suchthat an angle α is formed between an imaginary radial line through theouter edge of the upper section and a line formed by the top edge ofsaid upper section; and wherein said angle α is between about 20° to60°.
 9. The surface aeration impeller according to claim 8 wherein α andβ are from about 30° to 50°.
 10. The surface aeration impeller accordingto claim 9 wherein α is from about 30° to 45° and β is from about 35° to45°.
 11. The surface aeration impeller according to claim 9 wherein theheight of the outer edge of said upper section of said blades is between0.1 and 0.2 d.
 12. The surface aeration impeller according to claim 9wherein the height of the outer edge of said lower section of saidblades is between about 0.05 and 0.1 d.
 13. The surface aerationimpeller according to claim 8 wherein the height of the lower section isless than the height of the upper section.
 14. The surface aerationimpeller according to claim 9 wherein said blades contain only planer,non-curved segments.